1. Field of the Invention
This invention relates to the field of alternating phase-shifting masks, and in particular to a method of correcting three-dimensional (3D) effects in alternating phase-shifting masks using two-dimensional (2D) analysis.
2. Description of Related Art
To fabricate an integrated circuit (IC), a physical representation of the features of the IC, e.g. a layout, is transferred onto a plurality of masks. The features make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on. A mask is generally created for each layer of the IC. To create a mask, the data representing the layout for a corresponding IC layer can be input into a device, such as an electron beam machine, which writes IC features onto the mask. Once a mask has been created, the pattern on the mask can be transferred onto the wafer surface using a lithographic process.
Lithography is a process whose input is a mask and whose output includes the printed patterns on a wafer. As printed patterns on the IC become more complex, a need arises to decrease the feature size. However, as feature sizes shrink, the resolution limits of current optical-based lithographic systems are approached. Specifically, a lithographic mask includes clear regions and opaque regions, wherein the pattern of these two regions defines the features of a particular semiconductor layer. Under exposure conditions, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the lithographic process.
Various techniques have been proposed to improve this resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves. In this manner, the projected images from these two regions destructively interfere where their edges overlap, thereby improving feature delineation and allowing greater feature density on the IC. A mask that uses such techniques is called a phase-shifting mask (PSM).
In one type of PSM, called an alternating (aperture) PSM, apertures between closely spaced features are processed so that light passing through any aperture is 180 degrees out of phase from the light passing through an adjacent aperture. FIGS. 1A and 1B illustrate one embodiment of an alternating PSM 100 including closely spaced opaque (e.g. chrome or some other absorbing material) features 101, 102, 103, and 104 formed on a transparent, e.g. quartz, substrate 105. Thus, apertures 106, 107, and 108 are formed between features 101-104.
To provide the phase-shifting in this embodiment, the areas of substrate 105 under alternating apertures can be etched, thereby causing the desired 180 degree phase shift. For example, substrate 105 can be etched in the area defined by aperture 107 to a predetermined depth. In this manner, the phase shift of light passing through aperture 107 relative to light passing through apertures 106 and 108 is approximately 180 degrees.
Unfortunately, the use of a PSM can introduce an intensity imbalance problem. FIG. 1C illustrates a graph 130 that plots intensity (0 to 1.0) versus position on alternating PSM 100. In graph 130, waveforms 131 that are shown nearing 1.0 intensity correspond to apertures 106 and 108, whereas waveform 132 that is shown at approximately 0.84 intensity corresponds to aperture 107. The intensity imbalance between the 180 degree phase-shifting region (i.e. aperture 107) and the 0 degree phase-shifting regions (i.e. apertures 106 and 108) is caused by the trench cut into substrate 105, thereby causing diffraction in the corners of aperture 107 and degrading the intensity of the corresponding waveform. This industry-recognized diffraction effect is called a three-dimensional (3D) effect.
Intensity imbalance can adversely affect printing features and overlay on the wafer. Typically, a feature on a binary mask has a pair of corresponding phase-shifting regions on a PSM. For example, referring to FIG. 1D, a feature 140 can have a corresponding 0 degree phase-shifting region 141 placed relative to one side of feature 140 and a corresponding 180 degree phase-shifting region 142 placed relative to the other side of feature 140. Of interest, if phase-shifting regions 141 and 142 are the same size, the electric field associated with region 141 is stronger than the electric field associated with region 142, thereby resulting in the maximum interference of these fields to occur to the right of centerline 143 on feature 140. Thus, under these conditions, feature 140 will actually print on the wafer to the right of the desired location as shown by feature 150 and its associated centerline 153.
Moreover, any defocus in the system can exacerbate the 3D effect and cause significant deviation from desired feature placement on the wafer. Because any wafer production line requires at least some acceptable range of defocus, e.g. typically within 0.4 microns, feature placement is frequently adversely affected when using alternating PSM. Therefore, those in the industry have proposed various methods to address the intensity imbalance problem.
In one proposed method shown in FIG. 1E, an additional etching step can be performed on substrate 105, thereby providing an under-cut etch 160 of features 101-104. Under-cut etch 160 increases the intensity by attempting to localize the diffraction effects under features 101-104. Unfortunately, under-cut etch 160 can also create mechanical instability of features 101-104 on the mask. In fact, the more the diffraction effects are localized, the greater the probability of mechanical instability during subsequent processing steps, such as mask cleaning. Thus, under-cut etch 160 provides an incomplete solution with the potential of causing complete mask failure.
In accordance with one feature of the present invention, an accurate, cost-effective system and method for correcting three-dimensional effects on an alternating phase-shifting mask (PSM) is provided. To facilitate this correction, a method of building a library used for creating the alternating PSM can be provided. The method can include determining a first group of 180 degree phase-shifting regions, wherein the first group of 180 degree phase-shifting regions have a common first size. Three-dimensional (3D) simulation can be performed based on this first size. Of importance, a transmission and a phase can be altered in a 2D simulation based on this first size until a shape dependent transmission and a shape dependent phase allow the 2D simulation to substantially match the 3D simulation. Finally, a modified first size can be chosen using the shape dependent transmission and the shape dependent phase such that a 2D simulation based on the modified first size substantially matches the 3D simulation based on the first size. The library can associate the first size with the modified first size, the shape dependent transmission, and the shape dependent phase.
This method can be repeated for a plurality of groups of 180 degree phase-shifting regions for the alternating PSM, each group of 180 degree phase-shifting regions having a common size that is a different size than any other group. The size can refer to a width, a length, a width/length combination, or an area. In one embodiment, altering a transmission and a phase in the 2D simulation includes substantially matching a Fourier spectrum for the 3D simulation with a Fourier spectrum for the 2D simulation.
A method of designing a lithographic mask using this library is also provided. The method includes placing 0 degree phase-shifting regions and 180 degree phase-shifting regions on the lithographic mask. At this point, the library of pre-corrected shifters and matching simulation information can be accessed. Any 180 degree phase-shifting region having a size referenced in the library can be replaced with a corresponding pre-corrected shifter. The method can further include performing optical proximity correction (OPC) on the 0 degree phase-shifting regions and any pre-corrected shifters on the lithographic mask. In one embodiment, OPC can be performed using the matching simulation information, thereby ensuring that the 3D compensation provided by the pre-corrected shifters is retained.
Thus, an alternating phase-shifting lithographic mask that compensates for 3D effects can include a plurality of 0 degree phase-shifting regions and a plurality of corresponding 180 degree phase-shifting regions, wherein each 180 degree phase-shifting region has a size based on its corresponding 0 degree phase-shifting region. Therefore, a first set of the 180 degree phase-shifting regions includes a first bias and a second set of the 180 degree phase-shifting regions includes a second bias, thereby selectively compensating for 3D effects caused by the 180 degree phase-shifting regions. Note that any reference to 0 and 180 degree phase-shifting regions is relative, not absolute. In other words, the difference in phase between the two phase-shifting regions is approximately 180 degrees. Thus, 3 degree phase-shifting regions and 182 degree phase-shifting regions could also be used in the methods herein described.
A system that compensates for 3D effects on an alternating PSM is provided. The system can include an input interface for receiving a layout of the alternating phase-shifting mask and an output interface for providing a modified layout that compensates for the three dimensional effects. A memory in the system can also include a plurality of original sizes for 180 degree phase-shifting regions on the mask, a plurality of pre-corrected sizes, a plurality of transmission values, and a plurality of phase values. Each original size has a corresponding pre-corrected size, transmission value, and phase value. The corresponding transmission value and phase value allow a 2D simulation for the corresponding pre-corrected size to substantially match a three dimensional simulation for the original size. The system can further include a plurality of computer-implemented programs for generating the corresponding pre-corrected size, transmission value, and phase value. Finally, the system can include a processor for executing the plurality of computer-implemented programs.